There are many cases where it is desired to control the flow of mediums, such as air, from one portion of a structure to another by restricting the flow thereof, yet provide means to reduce the restriction of flow during emergency conditions such as those which might otherwise produce enough force to fail the intervening structure. An example of this is in the floor of a wide-bodied aircraft wherein the relatively large volume of the cabin space above the floor must be vented to the space underneath the cabin floor when the underfloor volume is rapidly decompressed. If improperly vented, the relatively small differential pressure which can be developed across the cabin floor multiplied by the relatively large floor area can generate sufficient force to fail the floor and do structural damage to the aircraft. This problem has been recently recognized by governmental agencies who have promulgated regulations requiring means in aircraft to assure that a decompression in any compartment in an aircraft caused by a hole of at least 20 square feet can be safely accommodated. An obvious expedient to meet the requirement in such aircraft is to reinforce the floor so that it can withstand decompression loads structurally. However, such a solution is impractical in that it requires extensive retrofit of aircraft presently in service and increases the weight of the aircraft without a corresponding increase in efficiency, thus raising the overall cost of the aircraft to fly.
The prior art solution of providing simple blow-out panels cannot be used in most instances because their operation can be triggered by passenger applied forces. Another solution to the problem can be seen in a patent application by Robert G. McIntyre and Otto J. Minnich entitled "Frangible Aircraft Floor," U.S. Pat. Ser. No. 578,479 now U.S. Pat. No. 3,938,764 which was filed 19 May 1975, and is assigned to applicants' assignee. It has been used to provide venting through a floor structure without decreasing the useful load-carrying capacity of the aircraft. However, frangible floors cannot be used in all applications and, therefore, variable area vents which actuate automatically and are resistant to the tampering of passengers have been required.